This invention relates to permanent magnet electric motors, and in particular to a self-starting permanent magnet motor capable of stopping quickly upon de-energization in any one of several predetermined angular orientations, and having a high starting torque so as to quickly achieve steady-state angular velocity upon energization of the motor.
Certain applications calling for the use of permanent magnet electric motors require that the motors be extremely responsive in terms of stopping quickly and accurately upon de-energization of the motor, and rapidly achieving a steady-state rotational velocity upon re-energization of the motor. For example, permanent magnet motors which operate magnetic disk drives in computer-related applications must stop quickly and accurately without substantial oscillation, and start with high acceleration. In addition, space is at a considerable premium in such motors, especially along the axis of rotation of the rotor element. Thus, such motors need to be thin in their axial dimension so as to fit compactly within the confines of the disk drive housing.
The aforementioned desirable characteristics, however, are not readily compatible with one another. The ability to stop the motor quickly and accurately requires, in general, high reluctance torque, while the ability to start the motor quickly requires an even higher electromagnetic torque to overcome the high reluctance torque. High electromagnetic torque may be obtained by maximizing the number and/or size of turns of wire in the energizing coils, but this exacts a penalty in the form of increased resistance or increased volume of the coils and thus of the motor as a whole, and especially tends to enlarge the axial dimension of the motor.
Stopping of the rotor element of a permanent magnet motor in a predetermined position or in one of a set of positions upon de-energization of the motor has usually been accomplished in the past by providing an annular gap of nonuniform reluctance between the rotor and stator. In a rotary permanent magnet motor, such nonuniform reluctance in the gap creates reluctance or cogging torque which varies cyclically with rotation, sometimes adding to and sometimes subtracting from net output torque. Such reluctance torque exists even though the motor windings are de-energized. The reluctance torque results from the fact that the permanent magnets, located on either the rotor or stator element, tend to attract the magnetically permeable core of the other element into a mutual rotational relationship where the nonuniform reluctance of the gap is minimized so as to create the greatest permanent magnet flux between the rotor and stator. The rotational relationships in which the stator and rotor tend to align as a result of reluctance torque to maximize the permanent magnet flux are called detent zones. (A similar torque, also producing detent zones in rotational relationships where permanent magnet flux is maximized, can be created by non-uniform magnetic strengths of the permanent magnets, as illustrated in Brown U.S. Pat. No. 4,438,362.)
Nonuniform gap reluctance has usually been created in the past by providing a winding core of magnetic material having teeth of different sizes defining nonuniform gap dimensions, that is, by having some teeth protrude from the core to a greater degree than others, or by having some teeth wider than others. The longer and/or wider teeth create gap regions of lower reluctance relative to the shorter and/or narrower teeth. The reluctance torque tends to align the rotor and stator in any one of a set of detent zones where the longer and/or wider low-reluctance teeth are aligned with the centers of the permanent magnet poles. Examples of such arrangements include Brailsford U.S. Pat. No. 3,264,538 and Muller U.S. Pat. No. 3,873,897 where a variable reluctance gap is provided by varying the radial protrusion of the teeth of a stator core such that the length of the gap, and therefore its reluctance, varies over the width of each tooth. The detent zones created by such a variable reluctance gap serve to stop the rotor element in predetermined positions upon de-energization of the coil. The positions of the detent zones are selected so that the rotor will be self-starting upon the re-energization of the coil. In other words, the detent zones are arranged so that upon the reapplication of energy to the coil, the rotor will receive electromagnetic starting torque which will accelerate it in the proper direction.
The foregoing designs, however, do not address the problem of maximizing the starting torque in order to move the rotor quickly out of a detent zone. As gap reluctance is made more nonuniform, the detent zones become more narrowly defined and the motor becomes less susceptible to oscillation on stopping due to increased reluctance torque, all of which is desirable. However, the higher the reluctance torque the higher will be the electromagnetic torque required to accelerate the rotor upon energization of the motor because, as the rotor begins to move out of a detent zone, reluctance torque opposing motion increases more rapidly. The requirement for higher electromagnetic torque could be satisfied by simply providing additional turns of wire for the energizing coil, but additional turns would require more volume and increase the axial dimension of the motor which, in some applications as described above, is unacceptable. Alternatively, additional turns of smaller wire could be provided without necessitating an increase in volume and axial dimension if the resistance of the winding is allowed to increase substantially. However, this would result in a corresponding increase in the power requirement and operating temperature of the motor, and a drop in efficiency, which are also undesirable.
Although nonuniform reluctance gaps have been used in combination with nonuniform coil winding structures, as exemplified by Chang U.S. Pat. No. 2,761,082, the nonuniform structures are designed for other purposes and do not cooperate with permanent magnets to maximize starting torque and provide oscillationfree, narrowly-defined detent zones for stopping.
What is needed, therefore, is a permanent magnet motor having narrowly-defined detent zones and a high starting torque, yet occupying relatively little volume, having a relatively small axial dimension and a relatively low winding resistance.